This invention relates to lighting systems which control the level of illumination of one or more lamps, such as in a stage lighting system or in other lighting applications where varying intensities of lighting are desired. In particular, this present invention relates to controlling the intensity of light from high-pressure-discharge lamps, rather than incandescent or low-pressure-discharge lamps such as those of the tubular fluorescent type.
If high-pressure-discharge lamps are used with a dimmer which is not specifically designed for such lamps, the performance is generally unsatisfactory. Solid-state dimmers generally control the portions of each half cycle during which voltage from an AC voltage source is supplied to the lamp load. High-pressure-discharge lamps can extinguish when the voltage remains off for a significant portion of a half cycle and the normal ballasting which is used typically will not reestablish the arc once it is extinguished. Several prior art systems have minimized this problem by the use of a ballast which has two portions, such that the series inductance can be changed by means of a solid-state switch, typically a switch of the so-called triac design. This can be accomplished with a parallel reactor portion and a solid-state switch in series with one of the reactor portions to disconnect it for a portion of each half cycle. This can also be accomplished with two reactor portions in series and a solid-state switch which shorts out one of the reactor portions for a part of each half cycle. U.S. Pat. No. 3,816,794 issued June 11, 1974 to Snyder is one example of such a system.
Because of the difficulties of control, typically only one or two discharge lamps are operated from a single dimmer. This leads to a relatively expensive dimming system because of the number of dimmers involved and also because of the expense of running conduits both for power and for low voltage control wiring to each fixture. In addition, problems generally have been encountered in obtaining a wide range of light intensities from discharge lamps. At the lower intensity end, the lamps are somewhat hard to control and tend to drop out, i.e. extinguish, and once having dropped out, the lamps are difficult to restart and generally require several minutes to cool prior to restarting. At the higher intensity end, difficulties are often encountered due to the trigger pulse to the solid-state switch arriving prematurely, i.e., before current reversal. Because the circuit is inductive, the current is lagging and premature trigger pulses will be ineffective. This results in no power being supplied during that particular half cycle with the result that either the lamp will blink or more probably will drop out and cannot be restarted until the lamp has cooled.